1. Field of the Invention
The invention pertains generally to closed loop fuel management systems utilizing exhaust gas composition sensors and is more particularly directed to a failure indication feature of such systems.
2. Prior Art
The utilization of electronic control for the regulation of the air/fuel ratio of an internal combustion engine has been increasing recently because of the precise regulation that can be achieved by such control. This precise control can be used to effect increased drive-ability, fuel economy, or the reduction of noxious emissions. Most advantageous systems today can accomplish all three of these desirous results in differing combinations.
The electronic control unit (or simply ECU) of today can be operated in an open loop mode or in a closed loop mode. The open loop mode provides a fuel schedule to which is applied a closed loop correction signal to increase the precision of the air/fuel ratio control. The closed loop correction signal is generally produced by an integral controller receiving an input from an exhaust gas composition sensor having bilevel switching signal. The exhaust gas composition sensor most widely used for such systems is a solid electrolyte zirconia tube inserted into the flow path of the combustion products of the engine. This type of sensor generates a voltage which is an indication of the difference in the partial oxygen pressures between the exhaust products and an atmospheric or other oxygen reference.
In this manner, if the exhaust products contain an abundance of oxygen, the sensor will output a low voltage level indication that not enough fuel was combusted with the previous air/fuel charge or a lean condition. If the exhaust products have an absence of oxygen, the sensor will output a high voltage level indicating too much fuel was combusted with the previous air/fuel charge or a rich condition. A steep transition between the high and low levels will occur when the air/fuel ratio passes through the stoichiometric point or a ratio of approximately 14.8:1.
One of the problems with a closed loop system is the failure of the exhaust gas composition sensor to function properly. If the sensor should fail or give a false indication of air/fuel ratio, the integral controller will continue to correct in the direction in which it was integrating until it reaches the limit of its correctional authority. Without a further sensor signal the system will lock up at this level. For systems using more than one integrator with differing authority levels and ramp constants, a lock up can create a significant problem. The last integrator in the cascade is, in many cases, capable of changing the air/fuel ratio of the engine .+-.25%. If this integrator were allowed to lock-up in either a full rich or full lean position, the emissions and driveability respectively would be drastically affected.
Therefore, some form of failure indication technique and system is needed to allow the system to break the closed loop and operate in open loop mode after an exhaust gas sensor failure. In the art some sensor failure and sensor ready detection systems for exhaust gas sensors of the zirconia type have been proposed. Many of the proposed systems inject a current into the sensor resulting in either an erroneous rich signal at low temperatures in the case of a continuous injection or a possible degradation of the sensor for an intermittent current injection after the sensor warms up.
It is known that erroneous signals occur from this type of exhaust gas sensor if they are taken before the sensor has a chance to warm up. The zirconia tube of the oxygen sensor is a solid electrolyte porus to oxygen ions and this transport process of the sensor increases with temperature as its impedance decreases. Thus, a failure indication system must know the system is up to temperature before starting the failure indicating sequence.
Normally, the exhaust gases of the engine heat the sensor to temperature quickly, but during idling or cold starting the time necessary to bring the sensor to operating temperature can be quite different than expected. Conventional prior art systems initiate detection of a sensor failure by waiting a fixed time period before beginning the failure detection sequence and closing the loop. The time period must be long enough to ensure for all the varying operating conditions that the sensor will be up to temperature. For most operational instances this period will be longer than necessary causing the engine to run in an open loop mode without the benefit of closed loop control and in exceptional cases the period may be too short causing a false or indeterminent failure indication.
One operational system improves the loop closing function by providing a clamping circuit which is operated in response to the actual conditions of the engine, but still has a fixed time failure detection sequence. It would be highly desirable to close the loop in response to the actual operating parameters of the engine and to provide a failure detection scheme for the exhaust sensor that is initiated at a point based upon the time it takes the sensor to rise to its operational temperature.